A recent advance in memory storage technology is the introduction of optical memory storage discs. In such an optical disc, a plastic substrate is provided. A thin metallic film is then positioned atop the plastic substrate by vapor deposition. This thin metallic film, generally consisting of two binary alloys such as lead selenium (Sb.sub.2 Se.sub.3) and bismuth tellurium (Bi.sub.2 Te.sub.3), is referred to as the writing layer. To write digital information onto this writing layer, a semiconductor laser beam is used. The laser beam changes the two binary alloys into a single four-element alloy. The laser beam's impact point on the writing layer, a dot, has a dimension that is generally less than one micron in diameter. The dot, therefore, has a reflectivity that is different from its surrounding. In reading this optical disc, another semiconductor laser beam is provided. When the reading laser scans this disc, it is capable of detecting the presence of the dot area since it has a different reflectivity than that of its surrounding.
The permanent alteration of the writing layer generally confines the use of the disc as a read only memory device. Thus, contributing to the name WRITE-ONCE/READ-MOSTLY (WORM) disc. In such a WORM disc, a difficulty is encountered when one wishes to correct even a single piece of information contained in the disc. To do so, a substantial portion, if not all, of the information must be rewritten. To minimize the necessity of rewriting the entire information contained in a disc, the present invention is utilized.